1. Field of the Invention
This invention relates to a lime slaking and grit removal process and system particularly useful in preparation of slaked lime for power plant effluent gas scrubbing. When hydrated lime is to be used as a gas scrubbant, a commercial lime product (calcium oxide) is purchased and slaked (to Ca(OH).sub.2) immediately prior to use. Because of the quantity of slaked lime needed for large scrubbing installations and the needed activity characteristics of such slaked lime, i.e. it is to be highly reactive with SO.sub.2, a very high recovery of slaked lime is necessary in the process along with good removal of grit in order to reduce abrasion in the scrubbing equipment. While the slaked lime is primarily usable in association with SO.sub.2 gas effluent scrubbing, the process and system may be utilized to prepare slaked lime for other applications.
2. Prior Art
A system representative of the prior art is seen in U.S. Pat. No. 2,086,379 (Clark) which shows the use of lime and water in a lime mix vessel wherein a portion of absorption liquor (from treatment of gas mixtures) containing absorbed SO.sub.2, and neutral salts of sulfuric acid are admitted to precipitate gypsum crystals, which are then removed by filtration. Subsequently the neutral salts are precipitated in a second vessel, treated with acid liquor and lime and the resultant liquor returned for use in the gas absorption system. Clark, however, does no lime slaking.
U.S. Pat. No. 3,170,770 (Bousman et al.), assigned to applicant's assignee predecessor company, is representative of a lime slaking apparatus, wherein lime and water are reacted in various compartments, the lime particles rabbled and raked over various surfaces and unreacted solids including grit removed by a classifier screw. Lime slaking occurs rapidly on the lime particle surfaces with rapid disintegration of reacted material, resulting in a milk of lime suspension. Use of the above apparatus with highly reactive lime, however, results in a viscous slurry of 10-20% solid concentration in which separation and removal of small grit of +10 to +100 mesh size is difficult. Primarily, the waste material conveyed upwardly and outwardly by the spiral are of a greater size than the +10 to +100 mesh material.
U.S. Pat. No. 2,904,401 (Booth) describes a method and apparatus for hydrating (or slaking) of lime which recognizes the presence of unwanted materials including grit, sand, stones, etc. and involves the formation of a thick "paste" product rather than the conventional more dilute slurry. Booth recites that by using less diluent, 2 pounds of water per pound of lime, that there is greater efficiency in utilization of power applied to the agitating means to break-up the lime lumps fed to the apparatus. Booth further incorporates a dilution chamber for diluting the formed paste with water so that a dilute suspension may be formed as may be required for utilization of the product. Foreign matter drops into a sump area in the dilution chamber, however, such foreign matter includes only those foreign particles too large or too heavy to be carried in the lime slurry (Col. 8, line 6). Thus Booth does not provide for removal of +10 to +100 mesh grit which would remain in the lime suspension exiting the device for use.
U.S. Pat. No. 4,261,953 (Gisler) is directed to a lime slaking apparatus, more particularly to a type of attrition machine to perform an attrition, scrubbing or abrading action on the lime particles being slaked so that new surfaces are exposed on the lime pellets so as to continue the reaction to completion. A plurality of cells are employed in which high-intensity rubbing and abrasion of the particles is provided to comminute the lime feed particles. Output of the slaking system is fed to the pool of a spiral classifier where larger lime particles including waste particles are separated and then conveyed to a ball mill for further comminution and to a screen separator for removal of waste material. Gisler states that use is made of a much higher than usual lime solids concentration but does not disclose actual values or whether the mixture is a paste or not.
In the removal of sulfur dioxide from power plant stack gas, certain scrubbing systems use a suspension comprising precipitated gypsum, calcium sulfite, and freshly slaked lime directly in the scrubber. For the system to be effective, the slaked lime slurry should be fresh, at a maximum solids concentration and as free of grit particles as possible. Also, a lime slurry made from highly reactive lime is more effective in removing the sulfur dioxide, possibly because the lime particles are smaller and thus have a greater surface area, which contributes to a faster reaction.